1. Field of the Invention
The present invention relates to a portable wireless terminal with a built-in antenna, and, more particularly, to an antenna matching device and method that allow the terminal to have steady radiation performance regardless of whether a folder of the terminal is opened or closed.
2. Background of the Prior Art
Recently, portable wireless terminals are becoming more popular, and terminals having various functions and designs are being introduced. Further, small-sized, slim and lightweight terminals that are being introduced are, at the same time, required to provide various functions in order to satisfy user's demand. Therefore, the design of the terminal is focused on reducing the terminal size while maintaining or improving the terminal's functions, to ensure the customer satisfaction.
Specifically, terminals having a rod antenna (i.e. whip antenna) and a helical antenna that are protruded outward from the terminal are easily broken when the terminal is dropped, and such antennas reduce the portability of the terminal. Therefore, a plate type antenna installed within the terminal is being widely used. The plate type antenna is also called a built-in antenna, internal antenna, or intenna, and includes for example, a Planar Inverted F Antenna (PIFA), a Folded Monopole Antenna (FMA), and so on.
Though an external antenna is easy to design owing to its sufficient gain, the built-in antenna is not easy to design because its gain will vary depending on the shape of the terminal. Specifically, the impedance of a built-in antenna will vary according to opening or closing (or sliding up or sliding down) of the folder, causing degradation in efficiency. That is, the radiation performance of the built-in antenna significantly changes according to the opening or closing of the folder. Such change in radiation performance occurs because an antenna impedance matching is designed based on the opened position of folder (or, for sliding type terminals, whether the slider is slid up).
FIG. 1 is a circuit diagram of a conventional π-type antenna matching device.
Referring to FIG. 1, a π-type matching circuit consists of passive elements of inductors L1 and L2 and capacitor C1. The π-type matching circuit is used for antenna impedance matching. A detailed description of the π-type matching circuit will be omitted because the circuit is well known to those skilled in the art. Specifically, the design of the π-type matching circuit is merely based on the opened position of the folder (or slid up position).
FIG. 2A is a graph showing VSWR of the device depicted in FIG. 1 when a folder is opened, and FIG. 2B is a graph showing VSWR of the device depicted in FIG. 1 when a folder is closed. The vertical axis of each graph denotes VSWR and the horizontal axis of each graph denotes frequency.
The π-type matching circuit of FIG. 1 has an L1 value of 5.6 nH, an L2 of value 1.0 nH, and a C1 value of 1.5 pF. Triangular markers 1 and 2 are used to denote VSWRs measured from a Global System for Mobile (GSM) terminal having upper and lower frequency limits of 880 MHz and 960 MHz. Triangular markers 3 and 4 are used to denote VSWRs measured from a Digital Cordless System (DCS) terminal having upper and lower frequency limits of 1710 MHz and 1880 MHz.
In the case of the GSM terminal, when the folder is opened (refer to FIG. 2A), the graph reads a VSWR of 2.3629 in a transmission line and a VSWR of 1.4791 in a receiving line. In the case of the GSM terminal, when the folder is closed (refer to FIG. 2B), the graph reads a VSWR of 7.0171 in the transmission line and a VSWR of 2.5504 in the receiving line, and a resonant frequency is out of the frequency range between 880 MHz and 960 MHz. Since the antenna impedance matching circuit is designed based on the opened position of a folder, the graph reads the higher VSWR, and resonant frequency range is out of frequency range when the folder is closed. Specifically, when the resonant frequency is out of the frequency range, the transmission and reception efficiency of the terminal is lower and an undesired mute state of the terminal exists in weak electric fields.
Accordingly, in conventional systems the antenna matching is carried out by one π-type matching circuit. Since the π-type matching circuit does not cover both the opened (slid-up) and closed (slid-down) positions of the folder (slider), the radiation characteristic of the antenna is biased to one position (e.g. the opened position of the folder). That is, the antenna radiation characteristic can not be optimized for both positions of the folder. Therefore, there is a demand for an antenna matching technology that enables the antenna to have steady radiation performance regardless of whether the folder is opened or closed.